Certain high performance coolness storage systems include a covered cylindrical tank with heat exchange tube spirals spaced one over the other and connected to inlet and outlet headers so that a phase-change material (PCM) in the tank may be alternately frozen and thawed by brine circulating through tubes immersed in the PCM. Such systems have been sold commercially for many years by Calmac Manufacturing Corporation of Englewood, N.J., U.S.A., under the trademarks LEVLOAD and ICE BANK and are described in U.S. Pat. Nos. 4,671,347, 4,954,278 and 5,054,298.
In a preferred form of these coolness storage devices the circulation of the brine through the heat exchange tubes is in a counterflow or opposite direction from one tube spiral to the next. This is achieved by having a first brine inlet header in a central zone of the tank free of the tubes connected through every other tube spiral to a first outlet header in an outer tank zone surrounding the bundle of tube spirals, and a second inlet header alongside the first outlet header in the outer zone connected through the alternate tube spirals to a second brine outlet header alongside the first inlet header in the central tank zone.
In the operation of these high performance coolness storage systems the PCM is alternately melted during a discharge cycle and frozen during a charging cycle. During the charging cycle ice builds up around each heat exchange tube spiral starting from the end adjacent the inlet header and progressing to the end adjacent the outlet header. With the counterflow arrangement of brine circulation in alternate heat exchange tube spirals, this results in progressively totally freezing the PCM at a uniform rate in all regions of the PCM in the tank affected by the tubes. A substantially solid block of ice is thereby formed around he tubes. During the discharge cycle the ice is melted from around the tubes in the same progressive fashion.
In other differing designs of coolness storage apparatus in the prior art, heat transfer efficiency is known to be enhanced by introducing air bubbles into liquid PCM in the bottom of the tank so that streams of bubbles rising upwardly gently circulate the PCM as it melts. These differing forms of coolness storage devices typically consist of rectangular tanks in which a zone of PCM in the bottom of the tank never freezes, and it is there in that always-liquid bottom zone that the air bubbles originate through perforated tubes connected to a source of pressurized air.
Application of air bubble systems to known spiral tube counterflow heat exchangers has not heretofore been notably successful. As a discharge cycle commences and ice begins to melt uniformly throughout that zone of the tank affected by the spiral tubes, there is a prolonged period when uninterrupted horizontal layers of ice are still present across the tank, including a continuous layer beneath the lowermost tube spiral above the always-liquid bottom zone where the air bubbler tube sends streams of bubbles upwardly. The increasing amount of water forming throughout the tank during that period cannot be reached and circulated by the air bubble stream because of the barriers presented by continuous ice layers. The air bubbles ineffectually stream upwardly through the always-liquid central or outer zones of the tank with no heat transfer efficiency enhancement on the growing volumes of water still enclosed within the melting ice. Not until the continuous horizontal ice layers begin to melt and perforate, which occurs substantially simultaneously throughout the tank in a properly designed counterflow system, do the air bubble streams begin circulating the water in the operative regions of the tank affected by the spiral heat exchange tubes.